1. Field of the Invention
The present invention relates to a solid adsorbent for an unsaturated hydrocarbon such as ethylene, propylene, 1-butene, 2-butene, 1-pentene, 2-pentene, 1-hexene, 2-hexene, cyclopentene, cyclohexene, butadiene, pentadiene, hexadiene cyclopentadiene, and cyclohexadiene. The present invention also relates to a process for separating an unsaturated hydrocarbon from a gas mixture containing the same, together with nitrogen, oxygen, methane, ethane, carbon dioxide, and hydrogen.
2. Description of the Prior Art
Unsaturated hydrocarbons such as monoolefins (e.g., ethylene) and dienes are the most important basic or fundamental raw materials in the chemical industry. These unsaturated hydrocarbons are produced by the pyrolysis of saturated hydrocarbons such as natural gases, refinery gases, and petroleum fractions. Furthermore, substantial amounts of unsaturated hydrocarbons are contained in off gases derived, as by-products, from fluid catalytic cracking and also in purge gases derived from various processes. These gases, however, contain unsaturated hydrocarbons together with nitrogen, oxygen, methane, ethane, carbon monoxide, carbon dioxide, and hydrogen. Furthermore, these gases contain 1000 to 20000 ppm of water. Accordingly, the unsaturated hydrocarbons must be separated from these gas mixtures in order to use the unsaturated hydrocarbons as raw materials for chemical synthesis.
Known processes for separating unsaturated hydrocarbons from gas mixtures include a so-called cryogenic gas separation process. In this process, a gas mixture is liquefied by cooling and is then fractionated at an extremely low temperature (e.g., -95.degree. C. to -140.degree. C., in the case of ethylene). However, this process also has disadvantages in that complicated refrigeration and heat recovery systems are required, the equipment is expensive due to the use of high-grade materials, the power consumption is large, and the separation of carbon monoxide and nitrogen is difficult. In addition, water and carbon dioxide contained in the gas mixture must be removed from the gas mixture in a pre-treatment apparatus so that the content thereof is less than 1 ppm, since clogging will occur in a low-temperature pipe line system when water and carbon dioxide are contained in the gas mixture.
U.S. Pat. No. 3,651,159 discloses that a toluene solution of aluminum copper(I) chloride CuAlCl.sub.4 can separate an unsaturated hydrocarbon such as ethylene from a gas mixture containing the same, by forming a complex with the unsaturated hydrocarbon. This process, however, has disadvantages in that, since the aluminum copper(I) chloride is strongly reacted with water to irreversibly lose its complex-forming capability, the separation capacity is gradually decreased with the increase in the gas treatment amount even where the gas mixture contains as low as 1 ppm of water, and the unsaturated hydrocarbon separation apparatus is corroded due to the hydrogen chloride derived from the reaction of the aluminum copper(I) chloride with water. This process has further disadvantages in that toluene vapor must be separated from the recovered unsaturated hydrocarbon in a separate step, since the recovered unsaturated hydrocarbon released from the toluene solution of the aluminum copper(I) chloride contains the vapor of the toluene solvent, and that the process using a liquid absorbent is disadvantageous when compared with the process using a solid absorbent, from the viewpoints of the various process limitations. Furthermore, since the absorbed solution contains aluminum chloride therein, a Friedel-Crafts reaction of the unsaturated hydrocarbon with toluene in the presence of the aluminum chloride catalyst occurs, as a side reaction, causing a loss of the unsaturated hydrocarbon and denation of the liquid absorbent.
Other various processes for separating unsaturated hydrocarbons from gas mixtures have been proposed. However, until now there has been no satisfactory process in the art for separating unsaturated hydrocarbons from gas mixtures, especially by using solid adsorbents.